Summar of Work: Functional genomics provides for the elucidation of structure and function of proteins encoded by human genes. We have developed a genetic-based approach to characterizing human gene function, termed phenotype disruption, which allows us to functionally identify human genes that may have a role in DNA and chromosome metabolism and genome stability. The approach relies upon assessing phenotypic impact of over-expressed human cDNAs in wild type and genetically sensitized microbial mutants on specific genetic endpoints. A genetic interaction between the expressed human cDNA and a specific microbial mutant can provide insight into the protein function in humans. This method led to the isolation of human cyclin A1 gene due to altered phenotype in a replication mutant and subsequent characterization of human cyclins A1 and A2. We have also determined a genetic background in which expressed hRad51p results in synthetic lethality. This is providing the opportunity for additional molecular characterization of hRad51p and its interactions with other human proteins which we have cloned into yeast vectors (eg. BRCA1 and p53). Recent work with the replication protein hFEN1 indicates that it is able to complement a yeast rad27 mutant deleted for the hFEN1 homolog, and interactions are being examined with rad51 mutants. We are currently assessing synergistic interactions between mutations in hFEN1 and yeast replication mutants where the analogous yeast double mutants are synthetic lethals. Overexpression of wildtype p53 and DNA binding domain mutants can inhibit wildtype yeast growth. Based on expression of p53 in various yeast mutants, the p53 effect requires the recombination/ repair gene RAD51. Moreover, unlike a rad52 mutant, expressing p53 in a rad51 mutant does not result in a synergistic sensitivity to methyl methane sulfonate. These data along with the observation that p53 reduces the level of interchromosomal recombination between inverted repeats in a wildtype strain support the hypothesis that p53 may play a role in recombination via a specific interaction with yRad51p.